ADS1174IPAPR datasheet(11/35 Pages) TI

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FREQUENCY RESPONSE

-1

-2

-3

-4

-5

-6

-7

-8

-9

-10

NormalizedInputFrequency(f /f

DA A)

0.45

0.47

0.49

0.51

0.53

0.55

-20

-40

-60

-80

-100

-120

-140

0.4

NormalizedInputFrequency(f

IN/fDATA)

0.2

0.6

0.8

1.0

-20

-40

-60

-80

-100

-120

-140

-160

InputFrequency(f /f

IN DATA)

0.02

-0.02

-0.04

-0.06

-0.08

-0.10

0.2

NormalizedInputFrequency(f /f

IN DATA)

0.1

0.3

0.4

0.5

0.6

ADS1174

ADS1178

SBAS373 – OCTOBER 2007

The digital filter sets the overall frequency response.

The filter uses a multi-stage FIR topology to provide

linear phase with minimal passband ripple and high

stop band attenuation. The oversampling ratio of the

digital filter (that is, the ratio of the modulator

sampling to the output data rate: fMOD/fDATA) is 64 for

both High-Speed and Low-Power modes.

Figure 2 shows the frequency response of the

ADS1174/78 normalized to fDATA. Figure 3 shows the

passband ripple. The transition from passband to stop

band is illustrated in Figure 4. The overall frequency

response repeats at 64x multiples of the modulator

frequency fMOD, as shown in Figure 5.

Figure 4. Transition Band Response

Figure 2. Frequency Response

Figure 5. Frequency Response Out to fMOD

These image frequencies, if present in the signal and

not externally filtered, will fold back (or alias) into the

passband, causing errors. Table 2 lists the degree of

image rejection versus external antialiasing filter

order. The stop band of the ADS1174/78 provides

100dB attenuation of frequencies that begin just

beyond the passband and continue out to fMOD.

Placing an antialiasing, low-pass filter in front of the

ADS1174/78 inputs is recommended to limit possible

high-amplitude, out-of-band signals and noise.

Table 2. Antialiasing Filter Order Image Rejection

ANTIALIASING FILTER

IMAGE REJECTION (dB)

ORDER

(f–3dB at fDATA)

Figure 3. Passband Response

111

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